Recently, energy saving has been actively promoted to protect the environment. For battery-powered portable equipment, such as mobile phones, digital cameras, and the like, energy efficiency is especially important to prolong battery life. Such portable equipment typically uses a switching regulator that includes an inductor because it is efficient and can be made compact.
Generally, the switching regulator is a constant voltage generator that converts an input voltage to a predetermined constant voltage in accordance with a control signal. There are mainly two methods to change an output voltage of such constant voltage generator, i.e., changing a reference voltage and changing a detection ratio of the output voltage.
Overshoot or undershoot in the output voltage may occur when the output voltage is changed rapidly. For example, the overshoot in the output voltage occurs when the output voltage changes from a low level to a high level, and the undershoot in the output voltage occurs when the output voltage changes from a high level to a low level. Such overshoot and undershoot in the output voltage should be avoided because the overshoot and undershoot may cause a load circuit, which is connected to the constant current, to malfunction.
In a known constant voltage generator that changes a detection ratio of the output voltage, a low-pass filter is employed and connected between an output voltage detector and an error amplifier for a predetermined time period from a start of a change of the output voltage. Accordingly, it is necessary to determine the time constant of the low-pass filter while taking into consideration a cross-over frequency of the constant voltage generator. If the time constant of the low-pass filter is too small, the overshoot or the undershoot in the output voltage may occur. By contrast, if the time constant of the low-pass filter is too large, although the overshoot or the undershoot in the output voltage may not occur it takes longer to reach a predetermined target voltage.
When the output voltage range is wide, ranging between a wide range between a low voltage lower than the input voltage and a high voltage higher than the input voltage, a step-down-and-step-up switching regulator is generally used. The step-down-and-step-up switching regulator has different cross-over frequencies for different operational conditions, i.e., a step-down operation and a step-down operation. If the input voltage is equal for both operational conditions, the cross-over frequency at the step-down operation is higher than the cross-over frequency at the step-up operation.
Thus, if the time constant of the low-pass filter is set to the cross-over frequency at the step-down operation, overshoot and undershoot in the output voltage occurs at the step-up operation. By contrast, if the time constant of the low-pass filter is set to the cross-over frequency at the step-up operation, it takes longer to reach a predetermined target voltage at the step-down operation.